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Stress rupture is an important failure phenomenon in composite overwrapped pressure vessels, which is highly unpredictable other than on a statistical basis. Even then, there are several statistical models, with varying bases in composite micromechanics and molecular failure mechanisms. Choosing among these models is not trivial, even when micromechanical details of the failure process are reasonably well appreciated, and one has available a reasonably large database of strength and lifetime data. As a result, there is little in the way of guidance to choose the most appropriate model. One important issue is that accurate predictions are desired at relatively low service loads compared to the strength, and low probabilities of failure that are far less, e.g., 10-6, than can be directly confirmed using the data itself. In essence, one needs a robust and accurate statistical model free of inconsistencies associated with such low stress levels and probabilities of failure. This paper performs an in-depth comparison of several current models, which have varying physical bases. The models compared differ in the number of parameters to be estimated from data. The results of this study, however, show that over a broad range of parameter values these models give surprisingly similar failure probability predictions. While practitioners may have a preference for one model over another, the basis for such a choice is not easily established, given the fidelity of typical data. [ABSTRACT FROM AUTHOR]

Multi-layered braided structures are formed as a result of over-braiding the previously formed braids and they are increasingly being used for numerous applications ranging from hoses to energy absorbing composites. In this research work, a series of multi-layered braided structures were prepared on circular braiding machine for obtaining various combinations of braid angles of 30° and 45° in inner and outer layers. Subsequently, the tensile properties of multi-layered braided structures were analysed and it was found that the braid angle in the outer layer has significantly affected the stress-strain behaviour. A simple analytical model for predicting the tensile behaviour of multi-layered braided structures has also been proposed based on the previously developed model of 'braid-elastic core' system. A clear distinction has also been made between the helix and braid angles. Furthermore, a comparison has been made between theoretical and experimental values of braid angle, toughness and stress-strain characteristics of multi-layered braided structures. [ABSTRACT FROM AUTHOR]